1. Field of the Invention
The present invention relates generally to compound semiconductor light emitting devices and, more particularly, to Group II-VI compound semiconductor light emitting devices which include at least one II-VI compound semiconductor quantum well layer disposed between II-VI compound semiconductor barrier layers.
2. Description of the Prior Art
Semiconductor lasers and light emitting diodes (LEDs) have a variety of applications including long distance fiber-optic communications, laser printers and compact disk players. Heretofore, these laser and LED devices were based on gallium arsenide (GaAs) and related Group III-V compound semiconductor materials, which operate at, and are limited to, infrared, and sometimes red, wavelengths. However, there are many advantages and potential applications for compact lasers and light emitting diodes at blue and blue/green wavelengths. Short wavelength semiconductor lasers would be extremely useful in optical data storage systems and display devices to increase information density which is inversely proportional to the square of the optical wavelength. For example, if a blue light semiconductor laser is utilized for an optical disk, the recording density can be increased. Also, the lack of a blue or blue-green laser or LED has made it impossible to complete the spectrum for high definition television (HDTV) systems. A further application of short wavelength lasers, such as a blue or blue-green laser, is a signal carrier for communications through sea water and ice since blue and blue-green light travel farther than other hues underwater. Other applications, both commercial and military, are also feasible. The basic advantage of blue and blue-green LEDs and lasers as compared to red and infrared, is the shorter wavelength, resulting in higher resolution as well as the increased sensitivity of the human eye to green and blue light.
Since semiconductor lasers and LEDs having shorter visible wavelengths than red and infrared have many advantages and practical applications, physicists, material scientists and electrical engineers have pursued the idea of extending these devices to shorter visible wavelengths since the 1960's. Approaches such as frequency doubling, GaN and SiC have met with only limited success. To date, the most promising medium for achieving emission at short wavelengths are direct bandgap II-VI semiconductor compounds with bandgap energies exceeding about 2.5 eV. The original concept was to make use of the direct bandgap recombination of such wide bandgap II-VI compound semiconductors as zinc selenide (ZnSe), zinc sulfide (ZnS) and zinc telluride (ZnTe).
There were two principal problems with such II-VI semiconductor materials: (a) achieving electrical control of the semiconductors by doping, and (b) the substantial defect state density within the bandgap due to impurities and defects, with doping often directly contributing to the latter. For example, it was possible to make some compounds p-type (such as ZnTe) and others n-type (ZnSe, ZnS) but it was impossible to obtain low resistivity in both polarities of a given compound, so the formation of a p-n junction was a difficult, almost impossible task.
Early efforts to develop II-VI technology primarily involved growing bulk crystals or films with equilibrium methods. These equilibrium grown crystals generally had large numbers of defects and significant concentrations of background impurities. About ten years ago non-equilibrium crystal growth techniques were investigated and the recent breakthroughs in wide-bandgap II-VI light emitters stem from molecular beam epitaxy (MBE), a non-equilibrium technique.
Early attempts to incorporate nitrogen by MBE were unsuccessful in obtaining p-type conductivity; neither were attempts to dope with P and As. The use of Li as a p dopant in MBE was somewhat more successful. Fundamental difficulties in the use of Li, however, included the tendency to incorporate Li interstitially (acting as an n-dopant) as well as the high diffusion coefficient of Li. Recently, Park et al., "P-Type ZnSe by Nitrogen Atom Beam Doping During Molecular Beam Epitaxial Growth", Appl. Phys. Lett., 57(20), pp. 212714 29 (1990) and Ohkawa et al., Japan J. of Appl. Phys., Vol. 30, p. L152 (1991) have obtained significant levels of p doping in ZnSe using a nitrogen rf plasma source. The use of the nitrogen plasma source has led to doping levels ranging from mid 10.sup.-17 cm.sup.-3 to 10.sup.18 cm.sup.-3 (comparable to the levels achieved in GaAs) resulting in the realization of pn junction light emitting devices operating in the blue and blue/green portion of the spectrum such as light emitting diodes and pulsed lasers. For example, Park et al., "P-Type ZnSe by Nitrogen Atom Beam Doping During Molecular Beam Epitaxial Growth", Appl. Phys. Lett., 57(20), pp. 2127-29 (1990) discloses blue LEDs based on p-type ZnSe epitaxial layers involving nitrogen atom beam doping during MBE growth. The LED includes a pn Junction of ZnSe formed on a GaAs substrate. An n-type cap layer is formed on the pn junction and an ohmic contact is formed thereon.
Concerning the second problem stated above, as a result of the presence of defects and imperfections, even when electrons and holes were introduced by external excitation (such as light or electron beams), the radiative yield of blue-green photons was very poor except at very low temperatures. In addition to these difficulties, not all of the compounds of interest possessed the same crystal structure (phase). A structure mismatch can be fatal when trying to construct layered heterostructure configurations such as those used in GaAs/(Al,Ga)As infrared lasers. (The notation (A,B) where A and B are elements of the periodic table corresponds to a chemical composition A.sub.x B.sub.(1-x) and will be used throughout the present specification. Accordingly, the notation (Al,Ga)As corresponds to a chemical composition Al.sub.x Ga.sub.1-x As).
Recent breakthroughs in the development of widegap II-VI light emitters allow fabrication of more complicated structures that include superlattice geometries with quantum well(s) to enhance the radiative recombination probability and the light emission of the devices. For example, Jeon et al., "Room Temperature Blue Lasing Action in (Zn,Cd)Se/ZnSe Optically Pumped Multiple Quantum Well Structures on Lattice Matched (Ga,In)As Substrates", Appl. Phys. Lett., 57(23), pp. 2413-2415 (1990) discloses optically pumped laser action in (Zn,Cd)Se/ZnSe multiple quantum well structures prepared by MBE on bulk (Ga,In)As substrates. Haase, et al., "Blue-Green Laser Diodes", Appl. Phys. Lett., 59(11), pp. 1272-74 (1991) discloses a blue-green laser diode which includes a (Cd,Zn)Se quantum well disposed between a pn junction of ZnSe. The device includes Zn(S,Se) cladding layers and is formed on an n-GaAs substrate. However, there are a substantial number of misfit dislocations (defects) as a result of the lattice mismatch between the Zn(S,Se) cladding layers and the ZnSe barrier layers. U.S. Pat. Nos. 5,081,632 and 5,045,894 disclose blue LED devices which include a ZnS.sub.0.65 Te.sub.0.35 QW formed between a homojunction of ZnS.sub.0.08 Se.sub.0.98. The devices are formed on GaAs substrates. U.S. Pat. No. 5,045,897 discloses a double heterojunction laser which includes a multiple quantum well (MQW) sandwiched between cladding layers of Zn(S,Se) formed on a GaAs substrate. The MQW includes alternating layers of II-VI quaternary Hg.sub.1-x Zn.sub.x S.sub.1-y Se.sub.y and ZnSe or Zn(S,Se).
Each of the above devices still suffer from problems such as strain related defects, inadequate operation at room temperature and device failure due to excess heating at the contact. With regard to the contact problem, conventional contacting to the p region (p-ZnSe top layer) was implemented by simply evaporating gold and then soldering a bonding wire to the gold using indium metal. The gold does not form an ohmic contact but instead forms a Schottky barrier with the p-region thereby causing a serious heating problem which can lead to device failure. Thus, the LEDs and diode lasers based on II-VI semiconductors that require a contact to a p-doped material (e.g., ZnSe) will not be practical without a means for forming low resistance ohmic contacts to the p-doped material.
Thus, there is a need to develop II-VI based semiconductor LEDs and diode lasers that emit light in the blue and blue-green portion of the spectrum and do not suffer from the above-mentioned problems. In addition, there is a need to develop a low resistance ohmic contact to such devices when they require a contact to a p-type material.